So Leigheas, we all have to carry the weight of our own decisions around--don't carry the weight of your sister's too. She made the choice to use. That's her burden to carry.
Easy to say, hard to let go of the guilt we feel, I know. Every day I think if I had been a better mother, more perceptive, more involved, kinder smarter better my daughter would be healthy today. Whipping myself over it doesn't help her, hurts me, sends me into a downward spiral and makes me less able to help her now. Does that resonate at all with you?
I hope you are able to let go of this, even a little.
It's not your fault.